If radiation dose is recalculated for each individual fraction and adjusted for intrafraction motion it is possible to estimate the real dose delivered to the target and compare this to the desired dose. This comparison gives a robust model of the potential benefit for real- time tracking and adjustment for the motion. Overview of kilovoltage intrafraction monitoring (KIM): Kilovoltage intrafraction monitoring is a novel real-time tumour localisation modality. It involves a single gantry- mounted kV x-ray imager (which is widely available on most linacs) acquiring 2D projections of implanted fiducial markers. As the treatment gantry rotates around the patient during treatment , the kV imager acquires 2D projections of the prostate . The fiducial markers are segmented using an in-house developed software package. 3D positions are determined via maximum likelihood estimation (MLE) of a 3D probability density.